Structures and methods for constraining spinal processes with single connector

ABSTRACT

Spinous process constraint structures include a first attachment element for placement over a first spinous process and a second attachment element for placement over a second spinous process. The attachment elements are joined by a single connector which may optionally include a compliance member for providing controlled elasticity between the spinous processes.

CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.12/426,167 (Attorney Docket No. 41564-703.502), filed on Apr. 17, 2009,which is a continuation-in-part of PCT Application US2007/081815(Attorney Docket No. 026398-000130PC), filed on Oct. 18, 2007, whichclaimed the benefit of Provisional Application No. 60/862,085 (AttorneyDocket No. 026398-000100US), filed on Oct. 19, 2006, the fulldisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The present invention relates generally to medical methods andapparatus. More particularly, the present invention relates to methodsand devices for restricting spinal flexion in patients having back painor other spinal conditions.

A major source of chronic low back pain is discogenic pain, also knownas internal disc disruption. Patients suffering from discogenic paintend to be young, otherwise healthy individuals who present with painlocalized to the back. Discogenic pain usually occurs at the discslocated at the L4-L5 or L5-S1 junctions of the spine (FIG. 1). Paintends to be exacerbated when patients put their lumbar spines intoflexion (i.e. by sitting or bending forward) and relieved when they puttheir lumbar spines into extension (i.e. arching backwards). Discogenicpain can be quite disabling, and for some patients, can dramaticallyaffect their ability to work and otherwise enjoy their lives.

This pain experienced by patients with discogenic low back pain can bethought of as flexion instability, and is related to flexion instabilitythat is manifested in other conditions. The most prevalent of these isspondylolisthesis, a spinal condition in which abnormal segmentaltranslation is exacerbated by segmental flexion. The device describedhere should as such also be useful for these other spinal disordersassociated with segmental flexion, for which the prevention or controlof spinal segmental flexion is desired.

Current treatment alternatives for patients diagnosed with chronicdiscogenic pain are quite limited. Many patients follow a conservativetreatment path, such as physical therapy, massage, anti-inflammatory andanalgesic medications, muscle relaxants, and epidural steroidinjections, but typically continue to suffer with a significant degreeof pain. Other patients elect to undergo spinal fusion surgery, whichcommonly requires discectomy (removal of the disk) together with fusionof adjacent vertebra. Fusion is not usually recommended for discogenicpain because it is irreversible, costly, associated with high morbidity,and of questionable effectiveness. Despite its drawbacks, however,spinal fusion for discogenic pain remains common due to the lack ofviable alternatives.

Recently, a less invasive and potentially more effective treatment fordiscogenic pain has been proposed. A spinal implant has been designedwhich inhibits spinal flexion while allowing substantially unrestrictedspinal extension. The implant is placed over one or more adjacent pairsof spinal processes and provides an elastic restraint to the spreadingapart of the spinal processes which occurs during flexion. Such devicesand methods for their use are described in U.S. Patent Application2005/02161017A1, published on Sep. 29, 2005, and having common inventorswith the present application.

As illustrated in FIG. 2, an implant 10 as described in the '017application, typically comprises an upper strap component 12 and a lowerstrap component 14 joined by a pair of compliant members 16. The upperstrap 12 is shown disposed over the top of the spinous process SP4 of L4while the lower strap 14 is shown extending over the bottom of thespinous process SP5 of L5. The compliant member 16 will typicallyinclude an internal element, such as a spring of rubber block, which isattached to the straps 12 and 14 in such a way that the straps may be“elastically” or “compliantly” pulled apart as the spinous processes SP4and SP5 move apart during flexion. In this way, the implant provides anelastic tension on the spinal processes which provides a force thatresists flexion. The force increases as the processes move furtherapart. Usually, the straps themselves will be essentially non-compliantso that the degree of elasticity or compliance may be controlled andprovided solely by the compliance members 16.

Ideally, the compliance members 16 will remain horizontally aligned andspaced generally between the spinous processes SP4 and SP5. In someinstances, however, the desired symmetry may be lost if the implantstructure 10 becomes circumferentially displaced about the spinousprocesses SP4 and SP5. Such displacement can affect the ability of theimplant to provide a uniform, symmetric elastic force to inhibit flexionof the spinous processes of a spinal segment in accordance with thedesired treatment. Also, the symmetric designs illustrated in FIG. 2 canbe difficult to deliver from the side which would be a preferredapproach in percutaneous delivery techniques.

For these reasons, it would be desirable to provide improved spinalimplants and methods for their use in inhibiting flexion in patientssuffering from discogenic pain. It would be particularly desirable ifthe improved devices would provide the desired elastic forces to thespinous processes with minimal risk of displacement or loss of symmetryof the device over time. It would be further desirable if the designsfacilitated percutaneous delivery from the side and other approaches.

Additionally, it would be advantageous if the implants and implantationmethods could be performed with minimum tissue disruption viapercutaneous and open surgical procedures. At least some of theseobjectives will be met by the invention as described hereinbelow.

2. Description of the Background Art

US 2005/0216017A1 has been described above. US 2006/0271055 describes aspacer having superior and inferior anchors and a spacer elementtherebetween. Other patents and published applications of interestinclude: U.S. Pat. Nos. 4,966,600; 5,011,494; 5,092,866; 5,116,340;5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722;5,462,542; 5,496,318; 5,540,698; 5,609,634; 5,645,599; 5,725,582;5,902,305; Re. 36,221; 5,928,232; 5,935,133; 5,964,769; 5,989,256;6,053,921; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309;6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975;6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140;6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205;Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458;US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US2004/0172132; US 2005/0033435; US 2005/0049708; US 2006/0069447;Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO02/051326 Al; WO 02/071960 A1; WO 03/045262 A1; WO 2004/052246 A1; WO2004/073532 A1; and Published Foreign Application Nos. EP 0322334 A1;and FR 2 681 525 A1.

SUMMARY OF THE INVENTION

The present invention provides spinal implants and methods forrestricting flexion of spinal segments for the treatment of discogenicpain and other spinal conditions, such as spondylolisthesis, where aphysician may desire to control segmental flexion. Systems according tothe present invention include spinous process constraint structurescomprising a first attachment element adapted to be placed over a firstspinous process, a second attachment element adapted to be placed over asecond spinous process, and a single connector joining the firstattachment element and the second attachment element. By “singleconnector,” it is meant that the connector joins a single point orlocation on the first attachment element to a single point or locationon the second attachment element. In contrast, the prior connectorsshown in FIG. 2, for example, provide a pair of connection points andtwo connectors for joining the upper component 12 to the lower strapcomponent 14. Use of the single connector for joining the first andsecond attachment elements reduces the likelihood that the attachmentmembers will become displaced such that the desired symmetric attachmentgeometry becomes asymmetric. A single connector also reduces the need tobalance the elastic forces being applied to the opposite sides of thespinous processes. The single connector will also simplify alignment ofthe implant during implantation, thus simplifying percutaneousimplantation and potentially minimizing tissue disruption in bothpercutaneous and other implantation protocols.

The single connector may comprise a single elastic member, where thesingle elastic member may itself comprise a continuous length of elasticmaterial having uniform or non-uniform elastic properties along saidlength. Alternatively, the connector may comprise an elastic memberincluding two or more separate components, for example inelastic ornon-compliant straps, cables, or other flexible members attached to acompliance member which provides the desired elasticity. Differentembodiments for the compliance members are described in co-pending,commonly owned application Ser. No. 12/106,103 (Attorney Docket No.026398-000410US), filed on Apr. 18, 2008, the full disclosure of whichis incorporated herein by reference. Regardless of the particularstructure, the single connector and/or elastic member will provide anelastic stiffness in tension between the attachment members in the rangefrom 7.5 N/mm to 50 N/mm, preferably from 10 N/mm to 25 N/mm, andusually in the range from 10 N/mm to 15 N/mm. In addition to providingsuch elastic stiffness in tension, the single connector and/or elasticmember will be constructed to provide little or no elastic stiffness incompression. Usually, the elastic stiffness in compression will be below3 N/mm, preferably below 0.5 N/mm. The ability of the constraintstructures of the present invention to provide a targeted elasticstiffness in tension while providing little or no elastic stiffness incompression allows for treatment of patient's having spinal segmentswhere the kinematics are improved by application of the elastic force tothe spine in flexion while providing little or no elastic resistance toextension.

The first and second attachment elements may have similar or differentgeometries. Exemplary geometries include open hook structures which maybe placed about the spinous processes and which have a single attachmentpoint for connection to the single connector. The attachment elementsmay also be loop structures which fully circumscribe the spinousprocess, where the loop is provided with a single connection point forconnection to the single connector. Often, the attachment elements willbe placed over the spinous process without further attachment. In otherinstances, however, it may be desirable to provide a secondaryattachment to the spinous process, such as staples, pins, screws,sutures, adhesives, energy-mediated attachments (such as laser welding),or the like. In some instances, one of the two attachment elements maybe adhered to the adjacent spinous process while the other of theattachment elements may be simply placed over the adjacent spinousprocess without adherence.

The constraint structures of the present invention may comprise separatecomponents which are joined or connectable together. For example, eachof the first attachment element, the second attachment element, and thesingle connector may be formed separately and interconnected byconventional techniques, such as screwing, welding, linking with maleand female attachment members, strapping, soldering, or any other suchfastening technique. In other instances, any two or more of thecomponents of the constraints of the present invention may be integrallyor monolithically formed from a common structural member. For example, apair of hook-like elements may be integrally formed with an intermediateconnector by forming the components from a single rod, wire, cable,polymer substrate, or the like.

The constraint structures of the present invention may be symmetric orasymmetric. For example, when loops or other attachment elementscircumscribe the spinous processes the connector may comprise a singleaxial member lying on the midline or mid-plane which bisects the spinousprocesses. Such a symmetric structure is advantageous since it appliesan axial force generally free from lateral components to the loops whichconstrain the spinous processes.

In other instances, however, it will be desirable to position the singleconnector on a side of the spinous processes so that the connector doesnot need to pass through the region between the spinous processes. Suchasymmetric constraint structures thus reduce or eliminate the need topenetrate the interspinous/supraspinous ligaments lessening patienttrauma and facilitating placement protocols. For such asymmetricdesigns, the attachment member may be a simple pin, screw, or otherfastener which penetrates the body of the spinous process, but will moreusually be a hook, loop, or other member which can attach to the spinousprocess without necessarily penetrating therethrough. For example, whenusing hooks, the upper attachment member can be placed over a superiorsurface of the superior spinous process while a lower hook member may beplaced around the inferior surface of the inferior spinous process.

When a single connector lies asymmetrically relative to the plane of thespinous processes, the connector will place the attachment members undera rotational load, often causing the single connector to bow inwardtoward the spinous process plane. Such deformation of the singleconnector will also tend to rotate and displace the attachment members,particularly those which are not fixedly attached to the spinousprocesses. In order to reduce such deformation and improve the stabilityof the spinous process constraints, a reinforcement member may be placedon or over the single connector, particularly within the region betweenthe spinous processes. For example, a reinforcement sleeve may be placedcoaxially over at least a portion of the single connector.Alternatively, and particularly when a compliance member is included inthe single connector (as described in more detail below) thereinforcement member may be a slide assembly which is attached to theconnector at a superior location and an inferior location and which canextend and contract together with elongation and contraction of thesingle connector while still maintaining alignment between the superiorand inferior segments thereof.

In another aspect of the present invention, the attachment members maybe hinged or pivotally connected to the single connector to facilitateintroduction and implantation of the constraint structure in a patient.For example, superior and inferior hooks may be pivotally attached atthe upper and lower ends of a single connector so that the hooks may befolded to reduce the profile of the constraint as it is being introducedinto position adjacent to the spinous processes. Once in position, thehooks or other attachment members may then be pivoted or otherwise movedinto place around the spinous processes to provide the desiredconstraint.

In yet another specific aspect of the present invention, the attachmentmembers may comprise clamps or similar structures which may be placedover posterior surfaces of the spinous processes to hold a singleconnector therebetween. Such posterior access is advantageous since itreduces the need to disrupt the/supraspinous ligament. Thus the use ofclamps or attachment members which are placed over the posterior surfaceof the spinous processes is particularly advantageous when used inconnection with an asymmetric single connector so that the penetrationof the supraspinous/ligaments is minimized.

The spinous process constraints of the present invention may furthercomprise a compliance member disposed within or as part of the singleconnector. The compliance member may have any structure which providesfor the desired elasticity in the connector to permit the first andsecond attachment elements to spread apart as the spinal segmentundergoes flexion. Suitable compliance members are described inpublished U.S. Application No. 2005/0216017 A1, which has beenpreviously incorporated herein by reference.

In other embodiments, the single connector may comprise an elastomericbody which is disposed between the first and second attachment elements.In some instances, the elastomeric body may be positionable over thesupraspinous ligament, and in certain of those cases such elastomericbodies may be adapted to be sutured or otherwise attached to thesupraspinous ligament.

In a further aspect of the present invention, methods for restrictingflexion of a spinal segment comprise positioning a first attachmentelement on a first spinous process and positioning a second attachmentelement on a second spinous process, wherein the attachment members arejoined by a single connector. The attachment members may be positionedin an open surgical procedure through the supraspinous ligament or maybe percutaneously implanted, optionally from a single sided posteriorapproach avoiding the need to penetrate the supraspinous ligament. In aspecific embodiment, the elements are joined with an elastic member,where the elastic member is preferably positioned over the supraspinousligament. In particular embodiments, the methods further compriseattaching the elastic member to the supraspinous ligament, for exampleby suturing. Usually, the methods further comprise penetrating thesupraspinous ligament to permit passage of the attachment element(s)and/or the elastic member therethrough. Still further optionally, theattachment members may be attached to the spinous processes, typicallyby stapling or any of the other attachment modalities described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 is a schematic diagram illustrating the lumbar region of thespine including the spinous processes (SP), facet joints (FJ), lamina(L), transverse processes (TP), and sacrum (S).

FIG. 2 illustrates a spinal implant of the type described in US2005/0216017A1.

FIG. 3 illustrates an exemplary embodiment of a spinous processconstraint structure constructed in accordance with the principles ofthe present invention.

FIGS. 4-11 are schematic illustrations of additional exemplaryembodiments of the spinous process constraint structures of the presentinvention, where the adjacent spinous processes are shown in section.

FIGS. 8A and 8B illustrate use of a reinforcement member on a singleconnector which does not include a compliance member, while FIGS. 9A and9B illustrate use of a reinforcement member on a single connector whichincludes a compliance member.

FIG. 12 illustrates a further alternative embodiment of a spinalconstraint structure of the present invention shown with a firstattachment member placed over the spinous process of L5 and a lowerattachment member attached to the sacrum.

FIGS. 13 and 14 illustrate yet another embodiment of a spinous processconstraint structure of the present invention where the attachmentmembers are placed over adjacent spinous processes with the singleconnector passing through and over the supraspinous ligament.

FIGS. 15A and 15B illustrate a spinous process constraint having a pairof clamps suitable for engaging posterior surfaces of a pair of adjacentspinous processes.

FIGS. 16A and 16B illustrate a connector similar to that shown in FIGS.15A and 15B which further includes a compliance member. In FIG. 16B,placement of the constraint adjacent to the supraspinous/interspinousligaments is illustrated.

FIG. 17 illustrates a further exemplary embodiment of a spinous processconstraint according to the present invention, shown with an asymmetricaxial member connected using pins to the spinous processes.

FIG. 18 illustrates a spinous process constraint similar to that shownin FIG. 17, further including a compliance member.

FIG. 19 illustrates a spinous process constraint according to thepresent invention having pivoted hooks for attachment of adjacentspinous processes.

FIG. 20 illustrates a spinous process constraint similar to that shownin FIG. 19, but further including a compliance member.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 3, a spinous process constraint structure 20constructed in accordance with the principles of the present inventioncomprises a first or upper attachment member 22 and a second or lowerattachment member 24. The first and second attachment members areconnected by a single connector 26, shown in the form of an elastic rodor cable. Usually, the attachment members 22 and 24 will benon-distensible, and will be firmly placed over the spinous processes,shown as the spinous process SP4 of IA and the spinous process SP5 ofL5. The connector 26 will be elastically distensible so that it willcomprise an elastic constraining force as a spinal segment undergoesflexion which causes the spinous processes SP4 and SP5 to spreadvertically apart. While being elastic in tension, the single connector26 will have a very low column strength so that it exerts very littleforce on the spinous processes SP4 and SP5 when the spinal segment is inextension and the processes move vertically toward one another. As usedherein, the phrase “spinal segment” is synonymous with the phrase“functional spinal unit (FSU)” and intended to mean the smallestphysiological motion unit of the spine that exhibits biomechanicalcharacteristics similar to those of the entire spine. A spinal segmentor FSU consists of two adjacent vertebrae, the intervertebral disc andall adjoining ligaments between them and excludes other connectingtissues such as muscles. The three joint complex that results issometimes referred to as the “articular triad.” Another term for the FSUis spinal motion segment. These definitions are taken from White AA,Panjabi MM. (1990), Clinical Biomechanics of the Spine, Philadelphia, JBLippincott.

The first and second attachment members 22 and 24 may be wrapped aroundthe associated spinous process SP4 and SP5 without further adherence orfastening. In some cases, however, it may be desirable to staple,suture, glue, or otherwise attach the attachment members to theunderlying spinous process. It will also be appreciated that in manyinstances the attachment members may have a seam or closure which allowsthem to be wrapped around the spinous process and closed in situthereover during an implantation procedure. It will be furtherappreciated that the single connector 26 may be preattached to either orboth of the attachment members 22 and 24. In other instances, however,it may be desirable to attach the connector 26 to either or both of theattachment members 22 and 24 during the implantation procedure in orderto permit the length of the connector to be adjusted. In particular, itwill be desirable that the length of the connector 26 be selected sothat the connector is generally fully extended but not under significanttension when the spinal segment is in its neutral (non-flexion andnon-extension) condition. In such cases, the connector 26 will begin toapply tension on the spinous processes 22 and 24 as soon as they beginto undergo flexion while collapsing and applying no force on the spinousprocesses as they undergo extension. FIG. 4 is a schematiccross-sectional view of the spinous process constraint structure 20 ofFIG. 3.

FIG. 5 illustrates an alternative spinous process constraint structure30 having first and second attachment members 32 and 34, similar tothose described in connection with FIGS. 3 and 4, and joined by a singleconnector 36 having a compliance member 38. In this embodiment, thesingle connector 36 may be formed from a non-distensible material wherethe desired elasticity is provided by the compliance member 38.

Referring now to FIG. 6, a spinous process constraint structure 40having a first or upper hook-like attachment member 42 and a second orlower hook-like attachment member 44 is illustrated. The first andsecond attachment members 42 and 44 are connected by a single contiguousor integral connector 46, which is transversely oriented in the spacebetween the upper spinous process SP4 and the lower spinous process SP5.The constraint structure 40 may be formed from a spring-like metal, suchas spring steel or nickel-titanium alloy, or alternatively may be formedfrom an elastomeric polymer. In some instances, the hook-like attachmentmembers could be reinforced or otherwise modified to be substantiallynon-compliant, while the connector 46 could be modified to enhance itselasticity, for example having a serpentine or coil spring structure.

Referring now to FIG. 7, a further spinous process constraint system 50comprises upper and lower hook-like attachment members 52 and 54 joinedby a single connector 56. The upper and lower attachment members 52 and54 as well as the connector section 56 may be formed from metal orpolymer and will typically be non-distensible. The desired elasticitybetween the attachment members is provided by a compliance member 58.

Referring now to FIG. 8, yet another spinous process constraint system60 comprises first and second hook-like attachment members 62 and 64.Instead of being connected in an S-shaped pattern, as shown in FIG. 6,the hook members 62 and 64 are connected in a C-shaped pattern, as shownin FIG. 8. Other aspects of the constraint system 60 may be similar tothose described with respect to constraint 40 of FIG. 6.

The spinous process constraint 60 of FIG. 8 will have a tendency todeform when placed under an axial load as the spinous processes undergoa flexion causing movement in the direction of arrow 65. Typically, aregion 66 of the constraint will tend to bow inwardly which causes thesuperior and inferior hook members 62 and 64 to displace laterally,increasing the risk that they will shift from their intended positionson the spinous processes. In order to alleviate this condition, areinforcement member 67 can be placed over a portion of the singleconnector 63 between the hooks 62 and 64. The reinforcement member maybe a simple sleeve constructed from a relatively rigid material, such asa metal or rigid polymer, having a central passage which is placed overthe single connector. Other reinforcement structures would also bepossible. Additionally, the sleeve embodiment shown in FIG. 8B could bemodified to be used with constraint embodiments including compliancemembers as described elsewhere in this application.

Similarly, as shown in FIG. 9, a spinous process constraint system 70comprises first and second hook-like attachment members 72 and 74arranged in C-shaped pattern, generally as shown in FIG. 8, furthercomprises compliance member 78 attached to superior and inferiorsegments of the single connector 76 (which is preferably non-compliant).Other aspects of the system may be generally as described in connectionwith the constraint structure 50 of FIG. 7.

The spinous process constraint 70 of FIG. 9 can also undergo deformationwhen subjected to an axial load, as shown in FIG. 9A. A reinforcementassembly 73 specifically adapted for constraints having compliancemembers 78 is illustrated in FIG. 9B. The reinforcement assembly 73connects to a superior segment 75 of the single connector 76 andincludes a slide rod 71 extending toward an inferior segment 77 of thesingle connector 76. The slide rod 71 is received in a bearing structure79 attached to the interior segment 77 which allows the rod to translateas the segments 73 and 77 move toward and away from each other as thespine undergoes extension and flexion. The reinforcement assembly 71helps maintain the proper alignment between the superior and inferiorsegments 75 and 77 to prevent the bowing and deformation illustrated inFIG. 9A.

In still another embodiment, a spinous process constraint system 80, asshown in FIG. 10, comprises a loop or encircling first attachment member82 and a loop or encircling second attachment member 84. The attachmentmembers 82 and 84 are joined by a connector 86 which, instead of beingattached at the center of the attachment members, is attached laterallyto one

side. It will be appreciated that the connector 86 could just as wellhave been attached laterally on the opposite side.

Referring now to FIG. 11, spinous process constraint system 90 comprisesupper and lower attachment members 92 and 94 which are similar to thosedescribed with respect to constraint structure 80 of FIG. 10. A singleconnector 96 is typically formed from a non-distensible material, andthe desired elasticity is provided by a compliance member 98 providedalong the length of the single connector 96.

As described thus far, spinous process constraint systems have beenintended to be placed on adjacent spinous processes. It will beappreciated that the constraint systems could be placed on spinousprocesses which are non-adjacent; e.g., separated by one or moreadditional spinous processes. It will be further appreciated that thespinous process constraint systems could be attached at a first or upperend to the spinous process SP5 of L5 and at a second or lower end to thesacrum S, as shown in FIG. 12. As the sacrum will often not include aprocess or other structure sufficient for attachment, when attachmentmember as described previously, spinous process constraint system 100may include a first or upper attachment member 102 similar to any ofthose described previously, and a second or lower attachment member 104which is modified to attach to the sacrum, e.g., by looping through ahole H formed in the structure of the sacrum. Other attachment memberssuitable for attaching to the sacrum are described in copendingApplication Ser. No. 11/827,980, filed on Jul. 13, 2007, the fulldisclosure of which is incorporated herein by reference. A singleconnector 106 is provided between the upper and lower attachment members102 and 104, optionally including a compliance member 108 to provide thedesired elasticity.

Referring now to FIGS. 13 and 14, yet another alternative spinousprocess constraint system and method for its implementation aredescribed. The spinous processes constraint system 110 includes a firstor upper attachment member 112 and a second or lower attachment member114. The upper and lower attachment members are joined by an elasticcomponent, typically an elastomeric body 116 which is configured to beplaced over the surface of the supraspinous ligament SSL, as shown inFIG. 14. The advantage of the constraint structure 110 is that it willminimally disrupt the structure of the supraspinous ligament, typicallyrequiring only minor penetrations to allow the placement of theattachment members 112 and 114. Optionally, the elastomeric body 116 maybe attached to the supraspinous ligament SSL, for example by sutures118, or adhesives, staples, or by other conventional attachment means.Similarly, because the elastomeric body 116 will be exerting a rearwardforce on the attachment members 112 and 114, it will typically bedesirable to staple, pin, suture, glue, or otherwise attach theattachment members to the spinous processes SP4 and SP5. While pins 120are shown, it will be appreciated that any of the other attachment meanscould also be used.

Referring to FIGS. 15A and 15B, a spinous process constraint structure140 comprises a superior clamp 142, an inferior clamp 144, and a singleconnector comprising an axial member 146 therebetween. The axial member146 may have any of the structures described previously to provide thedesired elasticity and modulation of flexion. The clamps 142 and 144 areformed so that they may be placed over the posterior surfaces PS of thespinous processes to be constrained, as shown in FIG. 15B. By employingclamps which are located over the posterior surfaces and furtheremploying a laterally displaced axial member 146 which is on the side ofthe spinous processes, the need to penetrate or otherwise disturb thesupraspinous and interspinous ligaments is minimized.

Referring to FIG. 16A and 16B, a spinous process constraint 150 having asuperior clamp structure 152 and an inferior clamp structure 154 isillustrated. The constraint 150 is similar to that illustrated in FIGS.15A and 15B, but further includes a compliance member 156 which joins asuperior segment 158 and inferior segment 160 of a single connectorbetween the clamps 152 and 154. Placement of the spinous processconstraint 150 on spinous processes SP4 and SP5 is illustrated in FIG.16B. The clamps 152 and 154 placed over the posterior surfaces PS of thespinous processes so that minimum intrusion is made into theinterspinous and supraspinous ligaments ISL/SSL. Similarly, as thesingle connector and compliance member 156 are on one side of thespinous processes, intrusion into the interspinous and supraspinousligaments is further reduced.

Other asymmetric spinous process constraint structures may beconstructed in accordance with the principles of the present invention.As shown in FIG. 17, for example, a single connector 180 may beconnected between pins 182 and 184 which are penetrated through thebodies of superior and inferior spinous processes. The single connector180 may also include a compliance member 182, as shown in FIG. 18.

As shown in FIGS. 19 and 20, a spinous process constraint may comprise asingle, continuous structure 200 having a superior hook 202 and aninferior hook 204. The superior and inferior hooks 202 and 204 arepivotally attached to an axial portion of the constraint therebetween.The constraint 200 may be introduced to a position laterally adjacent tothe spinous processes SP with the hooks 202 and 204 in retractedconfiguration. Once the constraint is in place on one side of thespinous processes, hooks 202 and 204 may be pivoted back over thesuperior and inferior surfaces of the spinous processes, as illustrated.A similar spinous process constraint structure 210 having a compliancemember 212 is illustrated in FIG. 20. Introduction of the constraintstructure 210 may be performed in the same manner as constraintstructure 200.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. It should be understoodthat various alternatives to the embodiments of the invention describedherein may be employed in practicing the invention. It is intended thatthe following claims define the scope of the invention and that methodsand structures within the scope of these claims and their equivalents becovered thereby.

1. A spinous process constraint structure for restricting flexion of aspinal segment, said constraint structure comprising: a first attachmentelement adapted to be coupled to a first spinous process; a secondattachment element adapted to be coupled to a second spinous process ora sacrum; and at least one connector joining the first attachmentelement and the second attachment element, said at least one connectorlying laterally beside the sides of the spinous processes and providinga connection between said attachment elements having an elasticstiffness in tension that resists flexion of the spinal segment withoutresisting extension thereof.
 2. A constraint structure as in claim 1,wherein the elastic stiffness in tension is in the range from 7.5 N/mmto 50 N/mm.
 3. A constraint structure as in claim 1, wherein the elasticstiffness in tension is in the range from 10 N/mm to 25 N/mm.
 4. Aconstraint structure as in claim 1, wherein the elastic stiffness intension is in the range from 10 N/mm to 15 N/mm.
 5. A constraintstructure as in claim 1, wherein said connector further provides anelastic stiffness in compression below 3 N/mm.
 6. A constraint structureas in claim 5, wherein said elastic stiffness in compression is below0.5 N/mm.
 7. A constraint structure as in claim 1, wherein the at leastone connector comprises an axial member positioned to lie parallel tothe sides of the spinous processes.
 8. A constraint structure as inclaim 1, further comprising at least one compliance member on the atleast one connector, wherein the at least one connector has a superiorsegment above the at least one compliance member and an inferior segmentbelow the at least one compliance member.
 9. A constraint structure asin claim 1, wherein the single connector comprises an elastomeric bodypositionable beside the supraspinous ligament.
 10. A constraintstructure as in claim 1, wherein at least one of the first and secondattachment structures is adapted to be fixedly attached to the spinousprocess.
 11. A constraint structure as in claim 10, wherein the firstattachment structure is adapted to be fixedly attached to a superiorspinous process and the second attachment structure is adapted to befixedly attached to an inferior spinous process.
 12. A constraintstructure as in claim 10, wherein the first attachment structure isadapted to be fixedly attached to a spinous process and the secondattachment structure is adapted to be fixedly secured to a sacrum.
 13. Aconstraint structure as in claim 1, wherein at least one of the firstattachment element and the second attachment element comprise a clampwhich is securable over a posterior face of the spinous process.
 14. Aconstraint structure as in claim 1, wherein at least one of the firstattachment element and the second attachment element comprise a pinwhich can be secured laterally through the spinous process.
 15. Aconstraint structure as in claim 1, wherein at least one of the firstattachment element and the second attachment element is adapted to beplaced around the spinous process without fixed attachment.
 16. Aconstraint structure as in claim 15, wherein the first attachmentelement is adapted to be placed over a superior surface of a superiorspinous process and the second attachment element is adapted to beplaced under an inferior surface of the interior spinous process.
 17. Aconstraint structure as in claim 15, wherein the first attachmentelement is adapted to be placed over a spinous process and the secondattachment element is adapted to fixedly secured to a sacrum.
 18. Aconstraint structure as in claim 16, wherein the first attachmentelement and the second attachment element are open hook structures. 19.A constraint structure as in claim 18, wherein the hook structures arepivotally attached to the at least one connector to allow closure of thehook over the spinous process after the at least one connector has beenaligned beside the spinous processes.
 20. A constraint structure as inclaim 18, wherein the at least one connector comprises an axial memberjoining the hooks in a C-pattern.
 21. A constraint structure as in claim1, wherein the at least one connector comprises a flexible element thatextends at least partly between the two attachment members, wherein theflexible member collapses under compression.
 22. A constraint structureas in claim 21, wherein the flexible member is compliant to provide saidelastic stiffness in tension.
 23. A constraint structure as in claim 21,wherein the flexible member is non-compliant under tension and locatedin series with a compliance member that provides the elastic stiffnessin tension.
 24. A method for restricting flexion of a spinal segment,said method comprising: positioning a first attachment element on afirst spinous process; and positioning a second attachment element on asecond spinous process or a sacrum; wherein the attachment elements arejoined with at least one connector extending therebetween and lyinglaterally beside the sides of the spinous processes, said at least oneconnector providing a connection between said attachment elements havingan elastic stiffness in tension that resists flexion of the spinalsegment without resisting extension thereof.
 25. A method as in claim24, wherein the at least one connector provides an elastic stiffness intension between said attachment elements in the range from 7.5 N/mm to50 N/mm.
 26. A method as in claim 24, wherein the elastic stiffness isin the range from 10 N/mm to 25 N/mm.
 27. A method as in claim 24,wherein the elastic stiffness is in the range from 10 N/mm to 15 N/mm.28. A method as in claim 24, wherein the single connector provides anelastic stiffness in compression between said elements below 3 N/mm. 29.A method as in claim 28, wherein said elastic stiffness in compressionis below 0.5 N/mm.
 30. A method as in claim 24, further comprisingattaching the attachment members to the spinous process and/or thesacrum.
 31. A method as in claim 24, wherein the attachment elements andelastic member are introduced percutaneously.
 32. A method as in claim31, wherein the attachment elements and elastic member are introducedlaterally from one side of the spinal midline
 33. A method as in claim31, wherein the attachment elements and member are introduced from aposterior approach from one side of the spinal midline.
 34. A method asin claim 24, wherein at least one of the first attachment element andthe second attachment element comprise a clamp positioning comprisessecuring the clamp over a posterior face of the spinous process.
 35. Amethod as in claim 24, wherein at least one of the first attachmentelement and the second attachment element comprise a pin and positioningcomprises securing the pin laterally through the spinous process.
 36. Amethod as in claim 24, wherein the attachment elements comprise hooksand positioning comprises introducing the hooks in a retractedconfiguration and pivoting the hooks relative to the at least oneconnector to capture the spinous processes after the at least oneconnector is positioned adjacent the spinous processes.
 37. A method asin claim 36, wherein a superior hook is pivoted over a superior surfaceof a superior spinous process and an inferior hook is pivoted over aninferior surface of an inferior spinous process.
 38. A method as inclaim 24, further comprising reinforcing the at least one connector tolimit deformation under axial load.
 39. A method as in claim 38, whereinreinforcing comprises placing a reinforcement sleeve over the singleconnector.
 40. A method as in claim 38, wherein the at least oneconnector includes a compliance member and reinforcing comprises placinga slide assembly adjacent to the compliance member, said slide assemblyaligning superior and inferior segments of the single connector.